Many methods are available for measuring humidity. An old and most commonly used method is the "sling psychrometer". This method uses two mercury thermometers, one of which has a wet sock on its bulb. The two thermometers are slung rapidly through the air so that the water in the wet sock evaporates, thus cooling the wet bulb to what's known as the wet bulb temperature, and so that the dry bulb is brought to normal air temperature. The wet bulb and dry bulb temperatures are then used to determine the humidity through well known calculations or simplified tables. A disadvantage of the "sling" method lies in the possibility that someone or something could get hit by the slinging psychrometer. A second disadvantage is the slinging time it takes for the water in the wet sock to evaporate and cool the wet bulb to a true wet bulb temperature. Often the user has to sling the psychrometer for such a long time that he does not have the time or energy to repeat the method for a double check.
A further disadvantage of the "sling psychrometer" is the limited accuracy of the method. The wet bulb temperature is measured on the inside of the wet sock instead of on the air-water interface side of the wet sock where there is a true wet-bulb temperature. The wet bulb of the sling psychrometer is actually cooled by a temperature gradient across the sock and consequently does not always achieve the true wet-bulb temperature.